The article has a range of excellent information, including tables of maturation points of major bones.

Below are a number of quotes from the article portraying the main points sought by the author, but I encourage you to read the article in its entirety.

“Believe it or not many vets are totally unaware, as many members of the general public are also unaware, that horses have more than one “growth plate”, that there are multiple ossification centers pertaining to every bone of the body outside of the skull, and that the schedule of growth-plate closure (which begins around the time of birth and extends until the sixth year, and is coordinated with the eruption schedule of the teeth) has been well known to veterinarians, paleontologists, zooarchaeologists, and mammalogists since the early 19th century.”

“There is no such thing [as a] slow-maturing breed. The Quarter Horse is not an ‘early maturing’ breed – and neither is the Arabian a ‘slow maturing’ breed. As far as their skeletons go, they are the same. This information comes, I know, as a shock to many people who think starting their colt or filly under saddle at age two is what they ought to be doing. “

“While growth in cannon bone length stops with the fusion of both growth plates at around 1 ½ years of age, increase in cannon bone girth does not taper off until close to 5 years of age, and essentially the same can be said for the girth of any other limb element, with those bones located higher up in the body maturing later.”

“Most of the growth plates above the distal radius in a three year old horse are unfused, including, most importantly, those of the animal’s spine. It is the spine of the horse that governs the overall coordination of the limbs and the animal’s running “style”. It is the spine, not the limbs, that the animal primarily uses to compensate for potholes, slick spots, and other irregularities in the race track [or any track]. The higher the speed and the greater the physical effort, the more important it is that the animal have all of its joints mature and in good working order. While catastrophic failures are uncommon, more subtle distal limb disease and chronic pain and dysfunction in two and three year old racehorses are commonly diagnosed and are major causes for the “wastage” of young Thoroughbreds.”

“What people often don’t realize is that there is a “growth plate” on either end of every bone behind the skull, and in the case of some bones (like the pelvis or vertebrae, which have many “corners”) there are multiple growth plates.”

“The lateness of vertebral “closure” is most significant for two reasons. One: in no limb are there 32 growth plates! Two: the growth plates in the limbs are (more or less) oriented perpendicular to the stress of the load passing through them, while those of the vertebral chain are oriented parallel to weight placed upon the horse’s back. Bottom line: you can sprain a horse’s back (i.e. displace the vertebral physes – see Figs. 5 and 8) a lot more easily than you can displace those located in the limbs.”